Thin film silicon semiconductor device and process for producing thereof

ABSTRACT

A thin film silicon semiconductor device provided on a substrate according to the present invention comprises a thin polycrystalline silicon film having a lattice constant smaller than that of a silicon single crystal and a small crystal grain size. This thin polycrystalline silicon film can be obtained by depositing a thin amorphous silicon film in an inert gas having a pressure of 3.5 Pa or lower by a sputtering deposition method and annealing the thin amorphous silicon film for a short time of 10 seconds or less to effect polycrystallization thereof. A thin film silicon semiconductor device comprising such a thin polycrystalline silicon film having a small lattice constant has excellent characteristics including a carrier mobility of 100 cm 2  /V·s or higher.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to a thin film silicon semiconductordevice which is important as a constituent element of athree-dimensional integrated circuit and a switching element for a flatpanel display or the like and a process for producing the same, andparticularly to a thin film silicon semiconductor device havingexcellent characteristics and a process for producing the same.

2. DESCRIPTION OF THE PRIOR ART

Thin film silicon semiconductor devices have recently attractedattention particularly as a constituent element of a three-dimensionalintegrated circuit and a switching element for a flat panel display, andhence are under extensive studies. Such semiconductor devices arereported in detail in a paper of D. S. Malhi et al. (IEEE Trans,Electron Devices ED-32 (1985) pp. 258-281). Thin film siliconsemiconductor devices as used in the above-mentioned applications, whenin the form of a field-effect transistor, comprise a thin silicon filmof 0.05 to 2.0 μm in thickness deposited as the basic constituent on aninsulating substrate. Among them, those having a coplanar structure or astaggered structure are most widely used. FIG. 1 is a cross-sectionalview of a thin film silicon semiconductor device having a coplanarstructure. This semiconductor device has a structure comprising aninsulating film 2 formed on the surface of an insulating substrate 1 anda series of a thin silicon film 3, a gate insulating film 4 and a gateelectrode 5 laminated thereon in this order, plus, sourceelectrode/drain electrode 6 for output power and metallic wiring 7. FIG.2 is a cross-sectional view of a thin film silicon semiconductor devicehaving a staggered structure. This semiconductor device comprises a gateelectrode 5, a gate insulating film 4, and a thin silicon film 3laminated in this order on an insulating film 2. Source electrode/drainelectrode 6 are formed on the gate insulating film 4. When a positive ornegative voltage is applied to the gate electrode 5, carriers areinduced in the inside of the thin silicon film 3, particularly near theinterface of the thin silicon film 3 with the gate insulating film 4,and flow between the source electrode and the drain electrode to developan output voltage between the metallic wiring 7, whereby this thin filmsilicon semiconductor device is actuated. As is apparent from the abovedescription, the characteristics of this semiconductor device aregreatly affected by the properties of the thin silicon film.

A thermal chemical vapor deposition method (thermal CVD method)basically comprising thermal decomposition of a gas comprising silane(SiH₄), disilane (Si₂ H₆), or the like as the main raw material, and aplasma chemical vapor deposition method (plasma CVD method) utilizingplasma and capable of easily effecting a treatment at a lower substratetemperature than the former method have heretofore been employed for theformation of a thin silicon film in a thin film silicon semiconductordevice. The plasma CVD method, which involves a lower temperature at thetime of film deposition than the thermal CVD method, is capable ofdepositing a thin silicon film even at 400° C. or below and hence allowsthe use of an inexpensive substrate such as glass, and is employed forthe production of a switching element for a flat panel display using aliquid crystal. Since a thin silicon film formed by this method isamorphous, however, the carrier mobility is as low as 1 cm² /V·s,resulting in a difficulty in obtaining a high-performance thin filmsilicon semiconductor device.

On the other hand, since the thermal CVD method involves a highersubstrate temperature than the plasma CVD method, it produces a siliconfilm not in an amorphous state but in a polycrystalline state. Since thesubstrate temperature in the thermal CVD method is 600° to 700° C. whichis by far lower than the melting point of silicon, of 1,412° C. however,the resulting silicon film is at best in a polycrystalline state of finecrystals. Therefore, the carrier mobility in a thin film siliconsemiconductor device produced by the thermal CVD method is at most about10 cm² /V·s, though the performance thereof is superior to that of athin film silicon semiconductor device comprising a thin silicon film inan amorphous state formed by the plasma CVD method. Thus, the thinpolycrystalline silicon film formed by the thermal CVD method also has agreatly limited scope of applications due to the substantially highersubstrate temperature and the comparatively low carrier mobility.

The reason for the poor characteristics of semiconductor devicescomprising a thin silicon film formed by the above-mentioned thermal orplasma CVD method resides in the amorphous or microcrystalline state,made of extremely fine crystal grains, of the thin silicon film formedby the above-mentioned method.

A common method for solving the above-mentioned problems and givingexcellent characteristics to the resulting semiconductor devicecomprises a heat treatment after the deposition of a thin silicon filmto grow crystal grains therein. Specifically, a thin silicon film isdeposited by a method utilizing a chemical reaction, such as the CVDmethod, and is subsequently crystallized by a laser annealing method oran annealing method comprising heating the film in a furnace at a hightemperature for a long time to effect the growth of into large crystalgrains having a size of 1 μm or larger. Even where a large crystal grainsize is provided by the above-mentioned method, the carrier mobility isat most 100 cm² /V·s, which cannot be said to be large enough to enablethe resulting semiconductor device to be applied to a wide variety offields.

The above-mentioned conventional thin film silicon semiconductor devicesfurther involve other problems in addition to the problem that highperformance characteristics cannot be obtained. For a thin silicon filmformed by the plasma CVD method, a trouble of the film exfoliating fromthe substrate frequently occurs. This results from the low substratetemperature at the time of film deposition which allows a large amountof unreacted silane gas, which is most widely used, hydrogen gas, andthe like to remain in the film, with the result that these gases arereleased from the film during the subsequent processing of the thin filmsilicon semiconductor device. By contrast, the thermal CVD method doesnot cause the trouble of the resulting thin silicon film exfoliatingfrom a substrate in the process for forming a thin polycrystallinesilicon film thereby and in the heat-treating of the resulting thin filmto provide a large crystal grain size. In this case, however, asubstrate incapable of resisting high temperatures (e.g., glass) cannotbe used for forming a thin film silicon semiconductor device thereon anda semiconductor device already formed the inside of a substrate isdeteriorated in characteristics or broken in the case of athree-dimensional integrated circuit because the substrate is exposed tohigh temperatures in the above-mentioned processes.

Further, a thin film silicon semiconductor device having comparativelyexcellent characteristics and comprising a thin silicon film havinglarge crystal grains involves the problem of a large lot-to-lotvariation of characteristics. This arises from the size of thesemiconductor device compared to the crystal grain size. FIG. 3 is aplan view of a channel region having a source electrode 8 and a drainelectrode 9. When crystal grains 10 are large as shown in the figure,the number and positions of crystal grain boundaries present in achannel region which boundaries hinder transportation of carriers differfrom semiconductor device to semiconductor device, resulting in a largelot-to-lot variation of characteristics. This is discussed in detailwith specific examples in, for example, a paper of K. K. Ng et al. (IEEEElectron Device Letters, EDL-2, 1981, pp. 316-318).

When a thin polycrystalline silicon film is processed by etchingaccording to a wet or dry process, crystal grain boundaries aresubstantially corroded as compared with the insides of crystal grains.In the case of a conventional polycrystalline silicon semiconductordevice comprising a thin polycrystalline silicon film having largecrystal grains, therefore, a difficulty is encountered in sharplyprocessing the sides of the pattern of the thin polycrystalline siliconfilm as shown in FIG. 3. Thus, it has been difficult to produce finethin film silicon semiconductor devices in high yield.

As described above, the conventional techniques involve the problemsthat it is difficult to obtain a high-performance thin film siliconsemiconductor device by a process involving a low temperature, and thatthe production yield is low.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a thin film siliconsemiconductor device having excellent characteristics by solving theproblems involved in the conventional thin film silicon semiconductordevice. Another object of the present invention is to provide a processfor producing a thin film silicon semiconductor device of the kind asdescribed above at a low temperature in high yields.

In accordance with one aspect of the present invention, there isprovided a thin film silicon semiconductor device disposed on aninsulating substrate which comprises a thin polycrystalline silicon filmhaving a lattice constant smaller than that of a single silicon crystalas the thin silicon film constituting the above-mentioned thin filmsilicon semiconductor device.

In accordance with another aspect of the present invention, there isprovided a process for producing a thin film silicon semiconductordevice on an insulating substrate which comprises the step of depositinga thin amorphous silicon film according to sputtering by glow dischargein an inert gas having a pressure of 3.5 Pa or lower, and the step ofannealing the thin amorphous silicon film for a heating time of 10seconds or shorter to effect polycrystallization thereof.

In the first aspect of the present invention, a thin film siliconsemiconductor device provided on as insulating substrate, comprises:

a thin polycrystalline silicon film having a lattice constant smallerthan that of a silicon single crystal as the thin silicon filmconstituting the thin film silicon semiconductor device.

Here, the ratio of the lattice constant of the thin polycrystallinesilicon film to that of the silicon single crystal may be 0.999 orlower.

The thin polycrystalline silicon film may have [111] axis orientation ina direction perpendicular to the surface of the substrate or in adirection close thereto.

The thin polycrystalline silicon film may contain boron in aconcentration of 10¹⁴ to 10¹⁷ /cm³.

The thin polycrystalline silicon film may contain as an impurity atleast one element selected from phosphorous and arsenic in a totalimpurity concentration of 10¹⁴ to 10¹⁷ /cm³.

The insulating substrate may be a glass substrate.

The thin polycrystalline silicon film may be formed through aninsulating film on the insulating substrate, and the device may furthercomprise a gate insulating film and a gate electrode formed in the ordernamed on the thin polycrystalline silicon film, and a source electrodeand a drain electrode may be formed on the first insulating film whilesandwiching the thin polycrystalline silicon film.

The thin polycrystalline silicon film may be formed on the substratethrough an insulating film, a gate electrode and a gate insulating film;and the device may further comprise a source electrode and a drainelectrode formed on the gate insulating film while sandwiching the thinpolycrystalline silicon film.

In the second aspect of the present invention, a process for producing athin film silicon semiconductor device on an insulating substrate,comprises the steps of:

depositing by sputtering a thin amorphous silicon film by glow dischargein an inert gas having a pressure of 3.5 Pa or lower; and annealing thethin amorphous silicon film to form a polycrystalline film and to have alattice constant smaller than that of silicon single crystal.

Here, the heating time of the annealing may be not longer than 10seconds.

The annealing may be effected by laser beam irradiation, electron beamirradiation or infrared ray irradiation.

The thin amorphous silicon film may be deposited on an insulating filmformed on the insulating substrate.

A gate insulating film may be formed on the thin polycrystallizedsilicon film, and a gate electrode may be formed on the gate insulatingfilm, while a source electrode and a drain electrode may be formed onthe insulating film.

The thin amorphous silicon film may be formed on a gate insulating filmformed through a first insulating film and a gate electrode on theinsulating substrate.

The gate insulating film may be formed so as to cover the insulatingfilm and the gate electrode, and a source electrode and a drainelectrode may be formed on the insulating film while sandwiching thethin polycrystallized silicon film.

The sputtering may be effected using a silicon target containing as animpurity at least one element selected from boron, phosphorus, andarsenic in a total impurity concentration of 10¹⁴ to 10¹⁷ /cm³.

The process for producing a thin film silicon semiconductor device mayfurther comprise the step of introducing as an impurity at least oneelement selected from boron, phosphorous, and arsenic in a totalimpurity concentration of 10¹⁴ to 10¹⁷ /cm³ into the thin amorphoussilicon film by ion implantation.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art thin film siliconsemiconductor device having a coplanar structure;

FIG. 2 is a cross-sectional view of a prior art thin film siliconsemiconductor device having a staggered structure;

FIG. 3 is a plan view of a channel region of a conventional thin filmsilicon semiconductor device;

FIGS. 4A to 4D are cross-sectional views of materials at subsequentprocessing steps in the production of a thin film silicon semiconductordevice according to the present invention;

FIG. 5 is a schematic cross-sectional view of a sputtering apparatusused for deposition of a thin silicon film according to the presentinvention;

FIG. 6 is a diagram illustrating drain voltage versus drain currentcharacteristic curves of a thin film silicon semiconductor deviceaccording to the present invention;

FIG. 7 is a diagram illustrating the relationship between the channellength and the carrier mobility in thin film silicon semiconductordevices according to the present invention;

FIG. 8 is an X-ray diffraction chart of a thin silicon film according tothe present invention;

FIG. 9 is a diagram illustrating variations of the carrier mobility andthe height of potential barrier developed in crystal grain boundarieswith the rate of change in the lattice constant of polycrystallinesilicon film in thin film silicon semiconductor devices;

FIG. 10 is a diagram illustrating variations of the carrier mobility andthe height of potential barrier developed in crystal grain boundarieswith the sputtering gas pressure at the time of deposition of a siliconfilm;

FIG. 11 is a diagram illustrating the relationship between the laserpower at the time of annealing by laser irradiation and the carriermobility in thin film silicon semiconductor devices produces by theprocess of the present invention;

FIG. 12 is a diagram illustrating variations of the carrier mobility andthe lattice constant with the annealing time; and

FIG. 13 is a diagram illustrating the relationship between the boronconcentration of a thin silicon film and the threshold voltage in thinfilm silicon semiconductor devices produced by the process of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 4A to 4D are cross-sectional views of materials at subsequentprocessing steps in the production of a thin film silicon semiconductordevice having a coplanar structure according to the present invention.FIG. 4A shows a layered structure made by subjecting substrate 12, of,for example, glass, to treatments such as polishing and washing,depositing thereon a first insulating film 13 such as a silicon nitridefilm or a SiO₂ film, depositing thereon a thin silicon film 14 bysputtering, and further depositing thereon a second insulating film 15(again an SiO₂ film or a silicon nitride film) having a thickness of0.05 to 1.0 μm for preventing contamination from the outside. FIG. 4Bshows the structure resulting from irradiating the thin silicon film 14with a laser beam to effect a short-time heat treatment therebyresulting in a polycrystalline state to the thin silicon film 14,processing the thin silicon film 14 by a customary photolithographytechnique and a customary etching technique, removing the secondinsulating film 15, and forming a gate insulating film 16 having athickness of 0.05 to 0.2 μm consisting of a SiO₂ film or a siliconnitride film. FIG. 4C shows the structure resulting from forming a gateelectrode 17 on the gate insulating film 16 by photolithography or thelike and introducing an impurity into the thin silicon film 14 to formsource electrode/drain electrode 18. FIG. 4D shows the structureresulting from forming metallic films 19 (Al or the like) as wiring byphotolithography or the like.

An apparatus used for depositing a thin silicon film is shownschematically in FIG. 5. This apparatus which includes a vacuum chamber20, an electrode 21, an electrode shield 22, a substrate holder 23, aheater 24, a gas-introducing inlet 25, and a gas-discharging outlet 26.A target 27 made of silicon is placed on the electrode 21, while asubstrate 28 is placed on the substrate holder 23. The vacuum chamber 20is evacuated. An inert gas is introduced into the vacuum chamber 20 fromthe gas-introducing inlet 25 and a negative DC voltage or ahigh-frequency voltage which usually has a frequency of 13.56 MHz isapplied to the electrode 21 to bring about glow discharge which servesto form ions. The ions formed collide with the surface of the target 27to strike silicon atoms and drive out them from the surface of thetarget 27. The silicon atoms thus sputtered are deposited on thesubstrate 28 to form a thin silicon film. The electrode shield 22 isprovided to avoid etching of the electrode 21 while allowing sputteringof only the target 27. The heater 24 is provided to enable the substrate28 to be heated to arbitrary temperature.

A thin silicon film was deposited on a substrate using argon as asputtering gas at an argon pressure of 2.0 Pa, a sputtering power ofR.F. 1.5 KW and a substrate temperature of 100° C. or lower. The thinsilicon film obtained by sputter deposition assumes a dense amorphousstate having a fibrous texture since the substrate temperature at thetime of sputter deposition is low and some silicon atoms alreadydeposited on the substrate are pushed into the film being deposited bysilicon atoms and argon atoms colliding therewith a high energyaccording to a knock-on phenomenon peculiar to sputtering. The internalstress of the deposited film as measured by the curvature of thesubstrate is a compressive stress.

An argon laser beam with an irradiation output of 2.8 W and a beamdiameter of 50 μl was scanned on the thin amorphous silicon film for anirradiation time of 1 millisecond on every part of the film beingirradiated therewith to anneal the film and to thereby convert the thinamorphous silicon film into a thin polycrystalline silicon film.Thereafter, an N-channel type thin film silicon semiconductor device wasproduced according to the process shown in FIGS. 4A-4D. FIG. 6 is adiagram illustrating the drain voltage versus drain currentcharacteristics of an N-channel type thin film silicon semiconductordevice as produced in the above-mentioned manner a different gatevoltages. The channel length and channel width of this thin film siliconsemiconductor device were 40 μm and 1,500 μm, respectively. The carriermobility found from this figure was 190 cm² /V·s. As can be seen in FIG.6, the thin film silicon semiconductor device according to the presentinvention shows good triode characteristics and has a large carriermobility.

FIG. 7 shows the carrier mobilities as measured in samples having afixed channel width of 10 μm and varying channel lengths. The otherproduction conditions were the same as the case of the sample as shownin FIG. 4. It can be seen from these results that the variation ofcarrier mobility was very small even when the channel length was varied.This indicates that, according to the present invention, the lot-to-lotvariation of characteristics of thin film silicon semiconductor devicescan be very much reduced even if the number of crystal grain boundariespresent in the channel varies from lot to lot, thus providing extremelyadvantageous conditions in actual designing and use of a semiconductordevice.

The crystal structure of a thin polycrystalline silicon film produced inthe above-mentioned manner was examined using X-ray diffraction. FIG. 8shows such an X-ray diffraction measurement of the thin silicon filmaccording to the present invention. It can be seen from this figure thata thin polycrystalline silicon film according to the present inventionis a [111] axis-oriented thin polycrystalline silicon film comprisingthe largest number of crystal grains having a [111] axis in thedirection perpendicular to the surface of the substrate or in adirection close thereto and a small number of crystal grains having a[220 ] axis or a [311] axis in those directions. That is, the siliconfilm has the largest number of crystal grains having a {111} face or aface close thereto which is parallel to the surface of the substrate.The diffraction peak intensity ratio (111)/(200) was about 5, which isconspicuously high compared to 1.8, as the intensity ratio of arandomly-oriented polycrystalline silicon powder. Further, it was foundthat the lattice constant of the crystal grain determined from thepositions of peaks was 5.411Å, which is smaller than 5.43086Å(see S. M.Sze: "Physics of Semiconductor Devices", John Wiley & Sons, New York;1969, P. 58), as the lattice constant of bulk silicon or a thin siliconfilm formed by the CVD method. The crystal grain size determined fromthe half value width of the (111) peak was about 0.06 μm. The thicknessof this thin polycrystalline silicon film was 0.16 μm.

It is known that the characteristics of a thin film siliconsemiconductor device comprising a thin polycrystalline silicon film areconspicuously affected by the properties of crystal grain boundaries,and that the lower the potential barrier developed in crystal grainboundaries, the higher the carrier mobility. FIG. 9 shows on therelationships of the carrier mobility (Curve A) and the height of thepotential barrier, determined from the dependence of drain current ontemperature, (Curve B) with the ratio of the lattice constant a/a₀ wherea represents the lattice constant of polycrystalline silicon film and a₀represents the lattice constant of a silicon single crystal in thin filmsilicon semiconductor devices. It can be seen from FIG. 9 that, as thelattice constant becomes smaller, the height of the potential barrierbecomes lower and the carrier mobility correspondingly becomes higher.With respect to thin film silicon semiconductor devices comprising athin polycrystalline silicon film having a lattice constant smaller thanthat of the single silicon crystal, the carrier mobility was as morethan 70 cm² /V·s despite fine crystal grains. Particularly when theratio of the lattice constant of the thin polycrystalline silicon filmto that of the silicon single crystal is 0.999 or less, the carriermobility was 150 cm² /V·s or higher. On the other hand, when a thinpolycrystalline silicon film having a lattice constant equal to orlarger than that of the silicon single crystal is used, a thin filmsilicon semiconductor device having excellent characteristics cannot beobtained.

As described above, the thin film silicon semiconductor device of thepresent invention has excellent characteristics despite a minute crystalgrain size, as compared to conventional ones. The fact that excellentcharacteristics can be secured despite a minute crystal grain size is ofgreat significance in producing thin film silicon semiconductor devicesin high yield. Crystal grain boundaries are conspicuously different notonly is electrical properties but also in processability compared to theinterior of crystal grains. Crystal grain boundaries are notablycorroded in processing a thin silicon film by a wet or dry etchingprocess, compared to the interior of crystal grains. This results in athin silicon film comprising large crystal grains shown in FIG. 3 aconventional thin film silicon semiconductor device. Thus, it has beendifficult to produce fine thin film silicon semiconductor devices inhigh yield. By contrast, according to the present invention, a thinsilicon having minute crystal grains can be sharply processed to enablea fine semiconductor device to be easily produced.

As described above, a thin polycrystalline silicon film for use in athin film semiconductor device having excellent performancecharacteristics is obtained when high energy particles collide withsilicon atoms already deposited on the substrate during the course ofdeposition of the thin silicon film to push some silicon atoms into theinterior of the film. Therefore, it is believed that the properties ofthe deposited thin film differ due to sputtering conditions. Statedanother way, a film of the kind described above can also be obtained bya vacuum evaporation and deposition method combined with irradiation ofthe film being deposited with high energy particles during the formationof the film. This method, however, requires a unit for emission andacceleration of high energy particles in addition to an evaporationsource, leading to a difficulty in deposition on a large-sized substrateand in uniform deposition. Such problems can be easily solved by thesputtering method.

FIG. 10 shows variations of the carrier mobility (Curve C) and of theheight of the potential barrier developed in crystal grain boundaries(Curve D) compared to the sputtering gas pressure at the time ofdeposition of thin silicon film. Argon was used as a sputtering gas fordeposition of a thin silicon film, and a laser annealing method wasemployed for polycrystallization under conditions of a laser power of2.8 W and an irradiation time of 1 ms. The deposited thin silicon filmsshowed [111] axis orientation irrespective of the argon pressure. As canbe seen from the figure, the lower the sputtering gas pressure, thelower the potential barrier and hence the higher the carrier mobility.Particularly when the pressure was 3.5 Pa or lower, the effect wasnotable. Particularly when the pressure was 2.5 Pa or lower, the carriermobility becomes 150 cm.sup. 2/V·s. With an increase in the sputteringgas pressure, the frequnency of collision of high energy particles withargon atoms increases to lose the energy of high energy particlescolliding with a substrate, resulting in a reduction in theaforementioned knock-on effect. Therefore, as shown in FIG. 10, thecarrier mobility notably lowered when the sputtering gas pressureexceeded 3.5 Pa.

The properties of a thin polycrystalline silicon film and thecharacteristics of a thin film silicon semiconductor device varydepending on annealing conditions for polycrystallization. Thinamorphous silicon films deposited by sputtering at an argon gas pressureof 2.0 Pa were polycrystallized by irradiation with argon laser beams atvarious irradiation powers, followed by the process shown in FIGS. 4A-4Dto produce thin film silicon semiconductor devices. FIG. 11 shows therelationship between the carrier mobility and the laser beam power inthe semiconductor devices thus produced. The irradiation time was onemillisecond in every case. It can be seen from these results thatcarrier mobility notably increases with an increase in the laser beampower.

Thin amorphous silicon films deposited by sputtering at an argon gaspressure of 2.0 Pa were respectively irradiated with an argon laser beamfor various irradiation times to effect polycrystallization thereof,followed by the process shown in FIGS. 4A-4D to produce thin filmsilicon semiconductor devices.

FIG. 12 shows variations of the carrier mobility (Curve E) and the rateof change in the lattice constant of thin polycrystalline silicon film(Curve F) compared to annealing time. When the annealing time was 50milliseconds or shorter, a laser beam irradiation method was employed.When the annealing time was one second to 600 seconds, an infraredirradiation method was employed. When the annealing time was longer than600 seconds, annealing was conducted in an electric furnace. The laserbeam irradiation was conducted by scanning a laser beam having adiameter of 50 μm and an irradiation output of 2.8 W. In the case of theinfrared irradiation, the whole surface of a thin silicon film wasirradiated with infrared rays to keep the surface temperature of thethin film at 1,200° C. In the case of annealing in the electric furnace,the temperature of the electric furnace was kept at 1,100° C. Annealingfor up to 10 seconds gave thin polycrystalline silicon films having alattice constant smaller than that of a silicon single crystal and thinfilm silicon semiconductor devices having a high carrier mobility. Whenthe annealing time was longer than 10 seconds, however, the carriermobility decreased.

As described above, a thin polycrystalline silicon film according to thepresent invention has a lattice constant smaller than that of a siliconsingle crystal, and conditions for obtaining such a thin polycrystallinesilicon film involve a pressure of 3.5 Pa or lower at the time ofsputtering and an annealing time of 10 seconds or shorter forpolycrystallization.

FIG. 13 is a diagram illustrating the relationship between the boron (B)concentration of a thin silicon film and the gate voltage for initiatingoperation of a semiconductor device comprising the thin silicon film,namely the threshold voltage. Here, introduction of boron into a thinsilicon film was carried out by ion implantation after the deposition ofthe thin silicon film by sputtering. The boron concentration is a valuecalculated from the amount of ion implantation and the thickness of thesilicon film. It can be understood from the results shown in this figurethat the threshold voltage increases with an increase in the boronconcentration, thus indicating that the threshold voltage can becontrolled by the boron concentration. When the boron concentration isless than 10¹⁴ /cm³ the introduction of the boron is not effective whilethe concentration is higher than 10¹⁷ /cm³, the resulting thin siliconfilm acts as a high concentration impurity-doped resistor incapable ofallowing a sufficiently high on-off current ratio to be secure, leadingto difficulty in operating a semiconductor device made of that material.Stated, another way, a method in which sputtering is effected using asputtering target as shown in FIG. 5 which contains a predeterminedamount of boron, that is 10¹⁴ -10¹⁷ /cm³, may be employed as the methodof introducing boron into a thin silicon film. Compared to the ionimplantation method, this method enables the introduction of boron to beeffected uniformly in the direction of growth of a thin silicon filmsimultaneously with the formation of the thin silicon film by the merepreliminary addition of a predetermined amount of boron to a target,thus advantageously leading to a reduction in the number of productionsteps.

The thickness of a thin silicon film for effectively embodying thepresent invention is most suitably in a range of 0.01 to 2.0 μm. When itis smaller than 0.01 μm, uniform annealing is difficult. When it exceeds20 μm, patterning of a thin silicon film by etching is so difficult thata semiconductor device having a thin structure cannot be produced.

Although argon was used as the sputtering gas in the foregoing examples,other inert gas such as helium, neon, xenon or krypton, or a mixture ofvarious inert gases may be used to obtain the thin amorphous siliconfilm having excellent characteristics. Although the laser beamirradiation method was employed as the short-time annealing method foreffecting crystallization of a thin amorphous silicon film in theforegoing examples, other methods including an electron beam irradiationmethod and an infrared irradiation method can be employed. The electronbeam irradiation method is suitable for production of a semiconductordevice and a three-dimensional integrated circuit on a glass substratesince it enables selective annealing to be effected only on and around athin silicon film while keeping a substrate at a low temperature. On theother hand, the infrared irradiation method, which uses light emittedfrom a halogen lamp or the like, is advantageous in that a thin siliconfilm can be annealed as a whole all at once. Application of either ofthe methods to a thin silicon film formed by sputter depositionaccording to the present invention can easily effect polycrystallizaitonof the thin silicon film to enable a thin film silicon semiconductordevice having excellent characteristics to be produces compared to thelaser beam irradiation method. When the annealing time forpolycrystallization exceeds 10 seconds, in the case of the infraredirradiation method for example, however, not only a thin silicon filmbut also the substrate is heated, and, in the case of athree-dimensional integrated circuit, lower-layer semiconductor devicesare deteriorated or broken. Thus, the annealing time must be 10 secondsor shorter.

The present invention can also be applied to both an N-channel type anda P-channel type thin film silicon semiconductor devices. Specifically,the two types of thin film silicon semiconductor devices as shown inFIGS. 1 and 2 respectively include an N-channel type using electrons ascarriers and a P-channel type using holes as carriers like a singlecrystal bulk semiconductor device. The former can be produced by addingphosphorus or arsenic as an N-type impurity to silicon in the sourceelectrode/drain electrode while the latter can be produced by addingboron as a P-type impurity to silicon in the source electrode/drainelectrode. The effective concentration of phosphorus and arsenic isequal to that of boron, i.e. and within the range from 10¹⁴ /cm³ to 10¹⁷/cm³. Phosphorus and arsenic can be introduced by sputter deposition.Although the description has been mainly made of coplanar structures inthe foregoing examples, the present invention can also be applied to astaggered structure. Specifically, a gate electrode is first formed, agate insulating film is then deposited, and thereafter a thin siliconfilm is formed, followed by relatively short annealing. Thus, a thinfilm silicon semiconductor device having a staggered structure can beproduced but only changing the order of the steps involved in theproduction of a thin film silicon semiconductor device having a coplanarstructure. It will be apparent that this provides a thin film siliconsemiconductor device having excellent characteristics.

As described above, a thin film silicon semiconductor device which hasexcellent characteristics despite a fine crystal grain size can beproduced by short annealing with the substrate temperature being keptlow. Therefore, an inexpensive substrate can be used and even a finesemiconductor device can be easily produced. Accordingly, the presentinvention produces high-performance thin film silicon semiconductordevices at low cost and in high yield.

The invention has been described in detail with respect to preferredembodiments, and it will now be apparent from the foregoing to thoseskilled in the art that changes and modifications may be made withoutdeparting from the invention in its broader aspects, and it is theinvention, therefore, in the appended claims to cover all such changesand modifications as fall within the true spirit of the invention.

What is claimed is:
 1. A thin film silicon semiconductor devicecomprising:an insulating substrate; a thin polycrystalline silicon filmdisposed on a surface of said insulating substrate, said thinpolycrystalline silicon film comprising fine crystal grains having alattice constant smaller than the lattice constant of a bulk singlecrystal of silicon; a gate insulating film disposed on said thinpolycrystalline silicon film; a gate electrode disposed on said gateinsulating film; and source and drain electrodes formed in respectiveportions of said thin polycrystalline silicon film, said gate electrodebeing interposed between said source and drain electrodes.
 2. A thinfilm silicon semiconductor device as claimed in claim 1, wherein saidthin polycrystalline silicon film contains boron in a concentration of10¹⁴ to 10¹⁷ cm³.
 3. A thin film silicon semiconductor device as claimedin claim 1, wherein said thin polycrystalline silicon film contains asan impurity at least one element selected from phosphorous and arsenicin a total impurity concentration of 10¹⁴ to 10¹⁷ /cm³.
 4. A thin filmsilicon semiconductor device as claimed in claim 1, wherein the ratio ofthe lattice constant of said fine crystal grains to that of said bulksingle crystal of silicon is within a range from 0.996 to 0.999.
 5. Athin film silicon semiconductor device as claimed in claim 1, whereinsaid fine crystal grains have a [111] axis oriented in a directionsubstantially perpendicular to the surface of said insulating substrate.6. A thin film silicon semiconductor device as claimed in claim 1,wherein said insulating substrate is made of glass.
 7. A thin filmsilicon semiconductor device as claimed in claim 1, wherein said gateinsulating film is composed of one of silicon oxide and silicon nitride.8. A thin film silicon semiconductor device as claimed in claim 1,wherein said source and drain electrodes are formed by doping said thinpolycrystalline silicon film with an impurity.
 9. A thin film siliconsemiconductor device comprising:an insulating substrate; a firstinsulating layer disposed on said substrate; a thin polycrystallinesilicon film disposed on said first insulating layer, said thinpolycrystalline silicon film comprising fine crystal grains having alattice constant smaller than that of a bulk single crystal of silicon,said thin polycrystalline silicone film having channel, source and drainregions, said channel region being interposed between said source anddrain regions; a second insulating layer disposed on said channelregion; and a gate electrode disposed on said second insulating layer.10. A thin film silicon semiconductor device as claimed in claim 9,wherein said channel region is doped with boron.
 11. A thin film siliconsemiconductor device as claimed in claim 9, wherein said channel regionis doped with at least one element selected from phosphorus and arsenic.12. A thin film silicon semiconductor device comprising:an insulatingsubstrate; a first insulating layer disposed on a surface of saidsubstrate; a gate electrode disposed on said first insulating layer; asecond insulating layer disposed on said gate electrode and said firstinsulating layer; a thin polycrystalline silicon film disposed on saidsecond insulating layer, said thin polycrystalline silicon filmcomprising a large number of fine crystal grains having a latticeconstant smaller than that of a bulk single crystal of silicon, saidthin polycrystalline silicon film having a source electrode region, adrain electrode region and a channel region, the channel region beinginterposed between said source electrode region and said drain electroderegion.
 13. A thin film silicon semiconductor device as claimed in claim12, wherein said source electrode and drain electrode region are formedby introducing an impurity into said thin polycrystalline silicon film.